The so called rack number; i take it that word "rack", being a (idiomatic) term to call a storing place, is not an actual description, thus its initial does not refer to appear in markings themselves. Meaning to say that the letter R in your sword would rather mean Regiment, as we used to mark weapons in my neck of the woods or, in some cases, someone's (owner) name initial.
Now, the X is quite a challenge, as hardly this letter is the first in a name ... or thing. In the German dictionary no more than a dozen names start with a X, for one.
On the other hand, the initials No as engraved, meaning 'number' as thought, are the type of symbol used by english speakers. Others would use Nš or №
Could it be that this number was engraved after the sword was imported ? What country did you buy from?
And last, before you hit me. Is it the different angle in which you took the photos of either marking set or could it be that one inscription was engraved in a different period than the other; they seem to have a different font style.
... And no more nonsense .